Internal combustion engine (ICE) starts are typically facilitated by a starter motor. The starter motor provides an initial crank to the engine, driving piston movement, and thereby creating suction to draw in a fuel and air mixture to one or more engine cylinders. Cylinder combustion may then be initiated once fuel is present in the cylinders and the pistons are reciprocating. Products of combustion may be treated by a catalytic converter to reduce emissions before being emitted to the atmosphere. However, catalytic converters must be heated to a sufficient temperature in order to adequately process the unwanted products of combustion.
During the start of a cold engine, engine emissions may be particularly high before the catalytic converter warms up enough to be effective. Further, liquid fuel may not vaporize as readily at lower temperatures, leading to increased amounts of unburnt hydrocarbons in the exhaust. In some examples, the volatility of cold liquid fuel may be so low, that even after cranking from the starter motor, the engine may fail to start. As such, a richer than stoichiometric (14.7:1) air to fuel mixture may be provided to the engine during a cold start to promote combustion. However, starting the engine with a rich air/fuel mixture may reduce fuel efficiency, and may increase hydrocarbon emissions due to incomplete burning of the hydrocarbons.
To reduce emissions during cold starts, several strategies have been developed to enhance heating of the catalytic converter. As one example, the exhaust system of an engine may be equipped with an Electrically Heated Catalyst (EHC). The EHC employs resistive elements which heat the catalyst prior to starting the engine. Other attempts have been made to adjust the fuel injection amount, fuel injection timing, and spark timing to increase exhaust gas temperatures so that the catalytic converter is heated more quickly. Once example approach is shown in U.S. Pat. No. 5,482,017 to Brehob et al., where the spark timing may be retarded during an engine start to increase exhaust gas temperatures and exhaust system components such as a catalytic converter. As another example, a more lean air/fuel mixture than stoichiometric may be injected during an engine start to create an exotherm in the catalyst and increase catalyst temperatures.
Other strategies to reduce emissions during cold starts include attempts to enhance hydrocarbon combustion efficiency, so that the exhaust gas mixture leaving the engine cylinders and entering the exhaust system is more depleted of uncombusted hydrocarbons. For example, U.S. Pat. No. 5,894,832 discloses a heating element which may be included in the intake system of an engine to pre-heat the fuel and air mixture before being delivered to one or more engine cylinders for combustion. After being heated, the fuel and air mixture may more readily combust, leading to a more complete burning of the hydrocarbons.
However, the inventors herein have recognized potential issues with such systems. As one example, electrically heated catalysts are more expensive and complex than traditional catalytic converters. Further, engine starting may be delayed to allow sufficient time for the EHC to be heated. Similarly, heating elements included in the intake to pre-heat the fuel and air mixture, add cost and complexity to the engine system. Retarding spark timing and adjusting fuel injection parameters during cold starts may reduce fuel efficiency, and in some cases, may increase emissions while the catalytic converter is heating up. For example, increasing the exhaust gas temperature by retarding spark timing or injecting a lean air/fuel mixture may produce elevated levels of NOx.
In one example, the issues described above may be addressed by a method comprising prior to a cold start of an engine: sealing a fuel tank from an evaporative emissions control system and an air intake of the engine, operating a fuel pump of the fuel tank to generate vapors in the fuel tank, and in response to fuel vapor levels in the fuel tank reaching a threshold, initiating cylinder combustion and flowing fuel vapors from the fuel tank to an intake manifold of the engine. In this way, emissions during engine starts may be reduced by purging fuel vapors from the fuel tank and/or canister to the intake manifold during the engine start. As explained above, the amount of liquid fuel injected during the engine start may be reduced by providing a portion of the fuel budget desired during an engine start in the form of fuel vapor. Since fuel vapors may combust more readily than liquid fuel, especially at lower ambient temperatures, the combustion efficiency of the engine during the start may be increased. That is to say, a more complete burning of hydrocarbons is achieved during an engine start. In this way, fewer unburnt hydrocarbons may be exhausted by the engine, therefore reducing emissions during the engine start. Spinning the fuel pump prior to the engine start may not only generate vapors in the fuel tank which may be used during an engine start, but it may also increase the temperature of the liquid fuel in the fuel tank. Since fuel vapors may combust more readily than liquid fuel, the success rate of engine starts may be improved by purging fuel vapors from the fuel tank to the intake manifold during the engine start and by increasing liquid fuel temperature.
In some examples, the method may additionally include injecting a desired starting amount of liquid fuel to the engine, and in response to opening of a canister purge valve, reducing the amount of liquid fuel injected to the engine based on an amount of fuel vapors flowing to the intake manifold, where the amount of liquid fuel injected to the engine is inversely proportional to the amount of fuel vapors flowing to the intake manifold. Prior to an exhaust oxygen sensor reaching a threshold temperature, the amount of fuel vapors flowing to the intake manifold may be estimated based on fuel vapor levels in the fuel tank and a vacuum level in the intake manifold as estimated based on outputs from a pressure sensor coupled to the intake manifold. After the exhaust oxygen sensor reaches the threshold temperature, the amount of fuel vapors flowing to the intake manifold may be estimated based on outputs from the exhaust oxygen sensor, and an amount of fuel provided to the engine may be adjusted by adjusting one or more of a fuel injection amount and/or the amount of fuel vapors flowing to the intake manifold, where the amount of fuel vapors flowing to the intake manifold may be adjusted by adjusting the position of a canister purge valve.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.